The present invention relates to a voltage controlled power supply for use with spray coating systems wherein electrostatic forces are utilized to aid in the deposition of spray particles on articles to be coated.
Electrostatic spray coating systems are widely used for the application of spray particles on articles to be coated. Such systems normally include a spray gun for which coating material is issued in atomized form, and means, such as an electrode to which a unidirectional high voltage is applied, associated with the spray gun for imparting a charge to the atomized material and for establishing a potential gradient between the spray gun and the article so that electro-static forces aid in transfer of the material to the article. Such spray guns maybe of the automatic type in which the gun is mechanically mounted and articles conveyed therepast, or the gun may be hand held. In addition, with such spray guns atomizing of the coating material may be accomplished in several different ways, for example by using air or hydrostatic forces as an atomizing medium, or by feeding the coating material to a rotary member which forms the material into a film and moves the film to the edge of the member whereat atomization takes place.
Spray coating systems of the type referred to readily lend themselves to commercial use and are capable of greatly reducing the costs of spray painting through the avoidance of coating material losses and the reduction in other costs, including labor of coating. However, such systems suffer a limitation in that proper electrostatic atomization and efficient deposition of spray coatings require an electrostatic field established by relatively high voltages, for example on the order of 60,000 volts and more, which presents difficulties in respect of arcing which may occur between the spray coating systems and the articles to be coated or other nearby objects.
As many spray coating materials are inflammable, particularly when atomized in air, it has heretofore been necessary to maintain a minimum spacing between the spray gun and the article to be coated in order to avoid the likelihood of an arc of sufficient intensity to initiate a fire or explosion. For installations in which the spray gun is mechanically mounted and articles to be coated are moved therepast on a conveyor, the gun is maintained in a generally fixed position with relation to the articles and there is little difficulty in maintaining the minimum spacing therebetween. However, for spray guns which are of the hand held type and manually manipulatable, there is no means to ensure that the minimum spacing will be maintained.
Typical of prior art attempts to avoid arcing between a spray gun and an article is that disclosed in Juvinall et al U.S. Pat. No. 3,048,498. As disclosed therein, a high impedance is connected in circuit between the charging electrode and an output from a high voltage power supply, and causes the voltage at the electrode to decrease with decreases in distance between the electrode and the article. In addition, an inductive-capacitive network at an input to the power supply attenuates the value of an a.c. voltage at the input to progressively decrease the high voltage output from the power supply in response to increases in output current.
Although efforts of the type disclosed in the aforementioned Juvinall et al patent have proven successful in decreasing the incidence of arcing from the charging electrode itself, as a result of the strong electrostatic field significant charges nevertheless accumulate on other portions of the gun, and usually on the atomizing portion which is normally in close proximity with the electrode, and are capable of arcing to an object brought in proximity therewith. Although considerable effort has been made to control such arcing through the geometric design of the spray gun, such efforts have not proven successful and arcing continues to be a problem.